This invention relates to improvements in a method for the ratio controlled mixing of liquids. One aspect of the invention relates to a method using an electrical automatic control loop for each of at least two liquids arranged in a master-slave relationship between the control loops. An aspect of the invention is a method of automatic control using electrical feedback automatic control systems where the master system has a programmable velocity profile and provides input reference signals to each slave control system that are representative of reference velocity and the position of a movable mechanical element in the master system. An aspect is the method of continuing to feed liquids even after mixing has stopped. The invention has particular application to the feeding and mixing of reagents that include polyol and isocyanate in the liquid injection molding of articles made of polyurethane compositions in connection with which the present invention will now be described by way of example but not limitation.
Liquid feeding mixers having means to adjust the feed ratio of one or more liquids to another are known, as is taught in a large number of previously issued U.S. and foreign patents, of which U.S. Pat. Nos. 3,386,623 and 3,642,175 are generally representative. It is also known to feed such liquids using plunger or piston type metering pumps.
It is common practice in the prior art to feed the reagents for making polyurethane sequentially or one at a time into a mixing chamber. This, however, generates a number of difficulties such as requiring longer cycle time for molding operations and also requiring a longer shelf time or pot-life to avoid having the reagents set up in the mixing chamber. One of the features and advantages of the instant invention is to reduce cycle time by allowing the use of fast-reacting chemicals whereupon the cycle time can be substantially reduced.
The method hereof embraces feeding interreactable liquids from plural sources to a mixing chamber from when the mixed liquids are injected into a mold cavity and any associated runners and sprues (hereafter called cavity) for completion of reaction and curing comprising the steps of: providing first and second positive displacement expandable chamber liquid metering pumps; filling the chamber of each pump, respectively, with a predetermined quantity of liquid in excess of that moiety of said liquid required to fill said cavity with a reacting mixture from said mixing chamber; establishing an open path from the outlet of each pump to within said mixing chamber; feeding each respective liquid through its path and into said chamber from its respective pump until said moiety of each liquid has entered said mixing chamber; simultaneously ceasing said feeding and then diverting said excess of each liquid, respectively, to its source therefore and meanwhile injecting into said cavity all of the admixed liquids that have entered said mixing chamber.